This invention relates generally to color cathode ray tubes (CRTs) having a shadow mask and is particularly directed to a mounting arrangement for a shadow mask of the tension foil type in a color CRT wherein the mount for the mask is in fixed relationship to the CRT faceplate.
The use of a tensioned foil shadow mask in a color CRT affords many advantages over conventional domed shadow masks. Chief among these is a greater power handling capability which makes possible as much as a three-fold increase in video image brightness.
A shadow mask serves as a color selection electrode, or parallax barrier, ensuring that each of the three electron beams lands only on its assigned phosphor elements, or deposits. A large percentage of the electrons directed towards the phosphor screen deposited on the glass faceplate are intercepted by the shadow mask, which therefore heats up during CRT operation. The elevated operating temperatures can cause the mask to undergo small but significant changes in both size and shape, the latter commonly referred to as "doming."
In order to preserve the critical geometric relationship between the shadow mask aperture array and the phosphor screen required for a high level of color purity, thermally induced size changes of the shadow mask are conventionally accommodated by supporting the shadow mask with compensating spring arrangements which allow the shadow mask to move towards the screen with increased shadow mask size and away from the screen with decreased shadow mask size while maintaining transverse registry with the screen.
Thermally induced shadow mask surface shape changes are addressed in color CRT designs which employ tensed foil shadow masks, thus utilizing the shape consistency of a taut planar membrane. Such CRT designs require that some tension be retained in the foil throughout its operational temperature range and that at the same time the periphery of the foil shadow mask be precisely controlled relative to the phosphor screen on the glass faceplate. An approach which combines size compensating foil shadow mask support springs with a taut foil shadow mask is disclosed in van den Broek U.S. Pat. No. 4,748,370.
Another approach utilizes the shape stability of a foil shadow mask and at the same time eliminates the critical manufacturing problems and performance limitations associated with shadow mask support compensation devices which must function during operation of the CRT. Examples of a fixed mask mount construction can be found in U.S. Pat. Nos. 4,547,695 to Rath and 4,695,761 to Fendley, the latter of which is assigned to the assignee of the present application. In this approach, a foil shadow mask is tensed in a fixture and then welded to a rigid support structure bonded to a flat glass faceplate. The overall size of the foil shadow mask is retained in fixed relationship to the phosphor screen. This size stability translates to a position stability of all apertures comprising the foil shadow mask array. During operational electron heating of the foil shadow mask, some tension will be lost; but the foil shadow mask array apertures remain stationary provided only that the heating is relatively uniform across the array. As a result, all electron beams will land only on their assigned phosphor deposits without requiring compensation for the critical spacing between the foil shadow mask and the phosphor screen. This critical spacing parameter is commonly called the "Q-Distance", which for the combination of a flat shadow mask shape and a flat screen is the same at all locations.
Electron beam landing precision in such a structure requires sufficient foil shadow mask pretension to counter the relaxation encountered during operational heating of the foil and sufficient rigidity in the support structure to accommodate the variation in tension in the foil shadow mask.
The effects of overall heating of a shadow mask with respect to potential changes in its size and surface shape have been addressed. In this context, the use of low coefficient of thermal expansion mask materials is generally beneficial, since mask shape variations with temperature are minimized by such materials. For the same reason, low thermal coefficient materials also enhance color purity performance for the case of non-uniform heating of the shadow mask which occurs when a portion of the display raster is made very bright while the remainder is dark. This type of mask deformation can be detrimental with all shadow mask types and mounting arrangements. In general, degradation of color purity caused by localized shadow mask heating cannot be compensated for, only minimized. In the case of a foil shadow mask in a fixed mount, mask apertures in the hotter regions of the array will be slightly displaced in the direction of the colder region of the array.
The amount of pretension required in a foil shadow mask fixed mounting approach determines, to a large extent, the structure employed in such designs. The sectional properties of the foil shadow mask support structure required to provide a high degree of stiffness must be compatible with the requirements for bonding that structure to the glass faceplate. The footprint width of the structure necessary to maintain the tension and integrity of the bond must be evaluated in regard to the real estate available on the glass panel. In addition, if the support structure is nonmetallic, a foil shadow mask anchoring element must be bonded to the structure. Finally, weld integrity at the periphery of the foil shadow mask is directly related to the required mask pretension.
All structural considerations are ameliorated with reduced pretension requirements. The pretension is determined by the operational temperature difference between the foil shadow mask and the CRT glass as well as the thermal and mechanical properties of the foil shadow mask material itself. Due to electron interception, a foil shadow mask may rise in temperature in the order of 100.degree. C. during CRT operation, while the glass remains relatively cool. In this environment, a foil shadow mask made of cold rolled steel 0.001 inches thick will require a pretension in the order of 40 pounds per linear inch. A mask of the same thickness made of a very low coefficient of thermal expansion material, such as Invar, might require only about five pounds per linear inch if minimum tension alone were the consideration.
A fundamental problem exists in the fabrication and processing of color CRT's of the type utilizing a fixed mounting of a tensed foil shadow mask if the foil shadow mask material has a very low thermal expansion coefficient. Tube frit sealing and exhaust processing temperatures are in the order of 435.degree. C., a temperature which is attained by both foil shadow mask and faceplate glass during assembly and production processing. A tensed foil shadow mask affixed to a rigid mount on the glass envelope and which is comprised of a material such as cold rolled steel having a thermal expansion coefficient greater than glass will simply relax all pretension when the assembly is subject to the typical CRT processing temperatures. On the other hand, a pretensed foil shadow mask made of material such as Invar (Trademark for a nickel-iron alloy with low thermal expansion), having a thermal expansion coefficient only a fraction that of glass, can be strained beyond its elastic limit if rigidly mounted in a CRT envelope when subjected to high CRT processing temperatures. Furthermore, many low expansion materials such as Invar exhibit greatly reduced mechanical strength at the elevated CRT processing temperatures, further increasing the likelihood of the foil shadow mask array being permanently deformed out of specification.
The purpose of this invention is to provide an arrangement for mounting in a color CRT a tensed foil shadow mask comprised of a material having a lower thermal expansion coefficient than that of glass. The inventive mounting arrangement provides the advantages of a fixed non-compensating approach during CRT operation; yet prevents mask over stressing and possibly even allows for a reduction in foil shadow mask pretension during high temperature CRT processing. The foil shadow mask mounting arrangement of the present invention is particularly adapted for use with mask materials having low thermal expansion coefficients which allow for a reduction in foil shadow mask tension. This reduction in tension permits the use of thinner support rails and facilitates mask installation. The foil shadow mask is attached to a spacer structure mounted to the inner surface of the CRT's glass faceplate by means of a plurality of flexible, resilient mounting springs. The mounting springs allow the glass components of the CRT to expand during high temperature CRT processing without exceeding the foil shadow mask's tension stress limits, while maintaining the foil shadow mask in registration with the phosphor deposits following CRT assembly and during lower temperature CRT operation. In another embodiment, a monolithic bi-metallic spacer structure mounted to the faceplate's inner surface is also directly coupled to the foil shadow mask and is inflexible at CRT operating temperatures to maintain the foil shadow in registration yet flexes at higher CRT processing temperatures during manufacture to allow for glass faceplate expansion without exceeding mask tensile stress limits.